Preface

Recently, I have masterd the use of zig, and I accidentally saw an article about luajit’s FFI. FFI can call C function in lua, that inspired me, why not use zig? zig can compile dynamic library which not depend on libc, there will be no compatibility issues.

So, I think this is available, let me try it!

How to use FFI in luajit?

In luajit, FFI is an embedded module, we just can use this to import it:

local ffi = require("ffi")
ffi.cdef[[
  int printf(const char *fmt, ...);
]]

ffi.C.printf("Hello %s!\n", "world")

A brief snippet, aha ?

This code is for importing ffi and define a function prototype with ffi.cdef, then we use ffi.c.printf to call clib’s function called printf to print “Hello, world”.

ffi.c is a namespace for c standard library prototype, through it we can easily call C standard functions!

Advanced use

Now, we just can use C standard library, so how to use other library?

em… we need to use ffi.load :

clib = ffi.load(name [,global])

the parameter name is the dynamic library name, and when global is true, the symbol table will be loaded in global namespace. if name is a path, it will load dynamic library from the specified path.

OK, this is a brief usage for FFI.

Compile dynamic library in zig

the code in build.zig:

const std = @import("std");

pub fn build(b: *std.Build) void {
    const target = b.standardTargetOptions(.{});

    const optimize = b.standardOptimizeOption(.{});

    const lib = b.addSharedLibrary(.{
        .name = "zig",
        .root_source_file = .{ .path = "src/root.zig" },
        .target = target,
        .optimize = optimize,
    });

    b.installArtifact(lib);
}

Then, we just define a export function hello_world in zig:

const std = @import("std");

export fn hello_world() void {
    std.debug.print("Hello, World!\n", .{});
}

Compile it, we can get a file called libzig.so, copy it to lua’s directory.

FFI call zig

Now, we have a dynamic library compiled by zig!

Let us write lua code:

local ffi = require("ffi")
local myffi = ffi.load("./libzig.so")

ffi.cdef([[
void hello_world(); /* don't forget to declare */
]])

--- @type function
local hello = myffi.hello_world

hello()

Then use luajit to execute this code:

$ luajit main.lua
Hello, World!

Perfect, now we can easily use zig in luajit!